Asymmetric digital subscriber line (ADSL) and very high DSL (VDSL) systems (collectively referred to as XDSL), transmit data simultaneously in both directions on a single pair of wires. To do this successfully, the receiver portion of the interface must be able to separate the desired signals supplied by the far-end transmitter from the other signals being sent by the near-end transmitter. The method used to accomplish this separation is similar to that used in broadcast radio. The frequency spectrum is divided between the two transmitters such that they transmit in separate regions (bands) of the spectrum. A typical VDSL spectrum allocation is shown in FIG. 1. The near-end transmitter bands are designated “US” for upstream, and “DS” for downstream.
Successful transfer of data is complicated by the fact that the line over which the data is transmitted tends to be quite lossy. Hence, the transmitter needs to drive a relatively strong signal, while the receiver is looking for a relatively weak signal from the far-end transmitter. Separating the weak signal from the strong signal generated by the near-end transmitter can be difficult. The transmit path of an xDSL circuit can generally be separated into three stages, as shown in FIG. 2. The first stage is a digital signal processor 10 that generates a mathematically perfect digital representation 12 of the signal. Following this is a digital-to-analog converter (DAC) 14, which converts the signal to weak real-world electrical waveform 16. The final stage is an amplifier 18 (also known as a “line driver”), which amplifies the signal 20 to a level that will be detectable even after transmission over a long lossy line. There are often filtering stages in the path also, which can be considered part of the DAC 14 or line driver 18, and are not shown.
By a process known as intermodulation, any amount of nonlinear distortion in the DAC 14 and line driver 18, which all real circuits have to varying degrees, will damage the spectral purity of the transmitted signal. If this distortion becomes too large, it will pollute the bands of the spectrum reserved for the downstream signal and make it impossible for the receiver to recover the weak signal from the far-end transmitter. Since the line driver amplifier 18 is typically the largest consumer of power in a xDSL analog front end product, it is usually optimized to minimize power consumption. This invariably results in compromised distortion performance; hence the line driver 18 tends to be the limiting source of distortion in the path.
A power amplifier, such as the line driver 18, typically consists of a feed-forward amplifier chain 22 wrapped in negative feedback loop 28 to enhance linearity. This structure is depicted in FIG. 3. Assuming the feedback block 28 is perfectly linear (which is a reasonable approximation), then the distortion of the line driver 18 is a function of two parameters: the intrinsic distortion of the feed-forward amplifier chain 22, and the ability of the negative feedback 28 to suppress any distortion via excess loop gain. The more linear the amplifier chain 22 is, the less excess loop gain is required to suppress that distortion. While excess loop gain tends to translate into higher power, there are also limits to how much excess loop gain can be achieved in any given process technology without making the feedback loop unstable. Also, since it deals with larger signals, the output stage 26 tends to dominate the distortion of the amplifier chain 22. Hence, the more linear the output stage 26 is, the easier the rest of the loop becomes to design.
Power-efficient CMOS output stages typically have poor distortion performance. It is, therefore, desirable to reduce the intrinsic distortion of such an output stage, especially low-order distortion such as the 3rd order harmonic distortion.